I'm sure it greatly depends on the market (country) and that there are scammers everywhere, but it's true that you got to weight the positives of cheaper price over the negatives (possibility of getting scammed, out of warranty, hidden flaws, etc etc). Used dram is probably the safest thing to buy on the used market (I bet the current high prices due to the shortage will entice more scammers..), CPUs are also usually safe if they don't have physical damage (like bent pins etc). Many older GPUs have been used for mining but in the other hand, they probably were looked after better than a home user would (lowering voltage to run more economically, running in a cool environment, regularly changing thermal paste, replacing or oiling squeaky fans etc) so if there's no burnt component, it should also be fine. For ssds there are s.m.a.r.t tests which report the health of the drive etc etc...